Dengue virus (DENV) causes dengue fever, the most prevalent arthropod-borne viral illness in humans. The existing four serotypes of DENV (DENV1, DENV2, DENV3, and DENV4) cause an estimated 390 million infections and at least 500,000 cases of Severe Dengue per year, which is a vascular leakage syndrome that can result in hypotension, shock, and death. Currently, no specific therapy is available, and only one vaccine is approved for use in humans (Dengvaxia). This vaccine lacks efficacy in nave individuals and appears to promote disease in some recipients upon subsequent natural infection. This phenotype may be due in part to suboptimal serotype responses and the phenomenon of antibody-dependent enhancement (ADE) of DENV infection, where cross-reactive yet sub-neutralizing levels of antibody can promote virus entry and infection in myeloid cells expressing activating Fc? receptors. Accordingly, all of the current DENV vaccines in trials have a goal of developing balanced and potently inhibitory tetravalent responses against the four serotypes of DENV. Recently, a new strain of DENV (DKE-121) was isolated in Malaysia from an individual with Severe Dengue, with up to 38, 37, 38, and 12% amino acid difference between DENV1, DENV2, DENV3, and DENV4 in the envelope protein. This strain may represent a new DENV serotype (DENV5). The potential emergence of a new DENV serotype could have major implications for existing tetravalent DENV vaccine platforms, which in theory, might lack coverage. Given the possible significance of such a strain (and serotype), in this exploratory R21 application, we propose to evaluate in detail the serological relatedness of DKE-121 to other DENV serotypes using a panel of existing and newly-generated sera and monoclonal antibodies. In parallel, we will develop a new mouse model of DKE-121 infection, which will be of utility to the field for assessing antibody activity, antibody cross-reactivity, and pathogenesis in vivo. We hypothesize that the genetic differences between DKE-121 strain and existing DENV serotypes will result in sufficient loss of serum cross-neutralization to classify this strain as a new DENV serotype, which could have significant consequences for existing DENV vaccine platforms and trials if this or related strains emerged.